IfcPropertyTableValue is a property with a value range defined by a property object which has two lists of (numeric or descriptive) values assigned. The values specify a table with two columns. The defining values provide the first column and establish the scope for the defined values (the second column). An optional Expression attribute may give the equation used for deriving the range value, which is for information purposes only. 

• If the Unit attribute is not specified, than the unit is already implied by the type of IfcMeasureValue or IfcDerivedMeasureValue. The associated unit can be found at the IfcUnitAssignment globally defined at the project level (IfcProject.UnitsInContext).
• If the Unit attribute is specified, the unit assigned by the Unit attribute overrides the globally assigned unit.

The IfcPropertyTableValue only captures properties that can be expressed by a table with two columns. Use IfcPropertyReferenceValue with the PropertyReference being an IfcTable to express all those properties that require a table with tree or more columns.